Classification/Diagnostic characteristics The naked mole rat, known more formally as heterocephalus glaber, is a member of the chordata animal phylum. Getting more specific, within the chordata are chordates. Looking at all chordates in general first, they, at some point in their lives, possess pharyngeal slits, a hollow dorsal nerve cord, a postanal tail, and notochord. It is important to understand what all of these characteristics are and what function they hold in order to determine what animals are chordates. The pharyngeal slits are a number of slits that the pharynx develops around and these slits open up to allow water to exit and enter. These slits are often present at some developmental stage of chordates but are often lost in adults. The hollow dorsal nerve chord is a hollow dorsal chord that connects to the notochord. The postanal tail is a tail that extends beyond the anus and finally, the notochord is the most distinctive derived chordate trait. It is composed of a core of large cells with congested fluid-filled vacuoles which make it rigid but flexible. Now within the chordates there are three clades: the cephalochordates, urochordates, and the vertebrates. The naked mole rat is classified as a vertebrate and therefore it does not have pharyngeal slits. What also makes vertebrates unique from cephalochordates and urochordates is that a dorsal supporting structure replaces the notochord in vertebrates. This dorsal supporting structure is better known as the dorsal vertebral column, where the vertebrates take their name. Vertebrates all contain a body plan that can support large, active animals. There are four key features that characterize vertebrates. The first key feature is the anterior skull with a large brain. This anterior skull contains the brain and many sensory organs. The second key feature is the rigid internal skeleton supported by the vertebral column. The third key feature is the internal organs suspended in a coelom. The coelom is a body cavity that develops within the mesoderm. The fourth feature is a well-developed circulatory system, driven by contractions of a ventral heart. (1, pg 485-488)
Relationship to humans Both naked mole rats and humans are chordates. While they both at some stage in their development possessed a dorsal hollow nerve chord, a postanal tail, and notochord, a major, common characteristic that humans and naked mole rats share is that both of these species lose their notochords and later develop dorsal vertebral columns. Both the naked mole rat and humans are in fact vertebrates. They both possess the four key features all vertebrates share (mentioned in the section above) and allow for both the mammals to have similar body functions. (1, pg 487)Naked mole rats have the longest lifespan of all rodents, typically living up to 30 years; this means they are very resistant to most diseases, including cancer. Scientists are sequencing the naked mole rat genome in order to search for both disease resistant genes and genes that enable them to live for so long relative to other rodents. This studies done on naked mole rats can go to help provide new information for potential ways to further the lifespan of other species such as humans. (3)
Habitat and niche Naked mole rats are the most eusocial mammals, mammals that show an advanced level of social organization. Naked mole rat colonies live underground in elaborate tunnel systems which are dug up by the colony members. These tunnels can extend as far as five kilometers and include up to seventy to eighty individuals with only one reproductive female and a few reproductive males. The other colony members are the offspring of the single reproductive female and are sterile workers that dig and maintain the tunnels, guard against intruders, harvest food, and use their feces to feed the queen and her offspring. (1, pg 817)A large population of the naked mole rats species live underground in arid savannah grasslands near the equator. (8)
Typical underground, naked mole rat colony.
(14)
Predator avoidance One way naked mole rats are able to defend themselves from predators is use of their niches. Being able to build their colonies underground gives them an extreme advantage against predators as they are harder to find and harder to get to. Also, because naked mole rats live in groups, they are able to protect each other in a highly efficient way. In the large groups, the younger offspring tend to settle near their mothers as they protect the young. Another key defense technique is alarm calling. Naked mole rats are able to make a distinct noise that warns the other members of the group of a predator. However, this technique does have a high risk cost as the noise attracts attention to individuals and makes them very prominent. (1, pg 817-818)
Nutrient acquisition Because naked mole rats developed to have jaws, they gained an advantage in improving their feeding efficiency. Animals with jaws can grasp, subdue, and swallow large prey. Another benefit naked mole rats possess is their ability to chew their food. Being able to chew food aids in chemical digestion and improves naked mole rats’ abilities to extract nutrients from their food. Also, because naked mole rats are mammals, the females have mammary glands that are able to secrete a nutritive fluid that can feed newborns. (1, pg 488, 497)
Reproduction and life cycle The reproduction process for naked mole rats is very similar to that of humans. It requires a male and a female to mate and if successful, an offspring will result. In a large colony of naked mole rats though, only one female is reproductive and a few males are reproductive. This limits the variety of genetics within the colony as all of the offspring is from a single female. However, there are some benefits that come to individuals that cannot reproduce in the colony. When chances of individual reproduction is close to zero, an individual can best maximize its inclusive fitness by staying with and helping maintain the colony. (1, pg 817) Differences between breeding and non-breeding naked mole rats, including more exaggerated reproductive organs, are caused by hormone levels; higher lutenizing hormone levels allow the breeding individuals in the colony to become fertile. However, if a non-breeding individual were to leave the colony, their hormone levels will regulate and they could become fertile in as few as eight days. (5)
Growth and development Naked mole rats possess key features that are most apparent in the early developmental stages, especially in the larvae. The first key feature is dorsal hollow nerve, postanal tail, and notochord, mentioned in section one. Both the dorsal hollow nerve and post anal tail are retained in the adult naked mole rate but the notochord turns into the vertebral column which becomes the new primary supporting structure. (1, pg 485-487)A unique way to identify non-breeding naked mole rats from breeding naked mole rats, is that non-breeding naked mole rats are smaller than breeding naked mole rats for both sexes (6).
Integument Naked mole rats are often clearly recognized by the wrinkly, thin skin they have; they do not possess a hard exterior. (1, pg 817) Naked mole rats also do not possess fur, which leaves them with a lack of insulation. Naked mole rats make up for this though by having a thicker epidermal layer than regular mole rats. (6)
Naked mole rats are easily identified by their thin, wrinkly skin.
(13)
Movement Naked mole rats often remain within the colony they help to build. These colonies consist of up to seventy to eighty naked mole rats that are constantly moving about in order to complete various tasks. Naked mole rats have two arms and two legs and travel on all fours throughout the tunnels in their colonies. (1, pg 817)
Living in groups underground, naked mole rats possess specific roles that contribute to the functioning of the overall colonies.
(8)
Sensing the environment Naked mole rats have pain sensors in the form of the NaV1.7 ion channel similar to other mammals such as mice and humans. However, they differs slightly in that they possess three different amino acids than all other mammals, and their ion channels are made up of proteins from these amino acids. Naked mole rats are just as sensitive to sunlight and heat as mice are, but this slight variation in the NaV1.7 ion channel makes naked mole rats less sensitive to other stimulus, for example acid concentration. (4)
Gas exchange Naked mole rats tend to burrow which causes gas exchange with the surface to be poor. The animals have to deal with low amounts of oxygen and high amounts of carbon dioxide and exhaled water vapor. This is not catastrophic through, as naked mole rat brains can tolerate low or even no oxygen for 15-30 minutes. (2)
Waste removal Since naked mole rats have anuses, they are able to digest their food and then empty their waste through their exterior end. (1, pg 486)Nitrogen buildup in the blood is filtered by the liver, and urea is excreted out of the body. (11)
Environmental physiology (temperature, water and salt regulation) Since naked mole rats are mammals, they have sweat glands which are able to secrete sweat that evaporates and thereby cools an animal. This allows the naked mole rats to be able to regulate their internal temperatures given the temperature of their environment. (1, pg 497) Though naked mole rats do have sweat glands, they have less sweat glands and a thicker epidermal layer than regular mole rats in order to retain more heat since they lack fur as mentioned in the integument section. (6)Unlike most mammals, naked mole rats are essentially cold-blooded, meaning that their body temperature conforms to that of their environment. (9)
Internal circulation Because the naked mole rat is a vertebrate, it has a closed circulatory system. In closed circulatory systems, the blood vessels keep the circulating fluid (blood) separate from the fluid around the cells (interstitial fluid). The blood cells and large molecules are able to stay within the circulatory system, but water and some other solutes leak out of the capillaries, the smallest vessel. In all closed circulatory systems, the blood is confined within a continuous system of vessels, and the blood is pumped through those vessels by one or more muscular hearts. (1, pg 747)
Chemical control (i.e. endocrine system) The naked mole rat's endocrine system functions similarly to that of most animals. The endocrine system functions to send chemical signals throughout the body and to other organisms. Endocrine cells secrete chemical signals to induce responses in other cells. Groups of endocrine cells form an endocrine gland. The chemical signals secreted by these cells are called hormones. Hormones secreted from a cell travel through the bloodstream and bind to the target cells. The reactions induced by specific hormones may induce short term responses, such as the fight-or-flight response, or permanent changes, such as the production of reproductive organs. (10)
Review Questions 1. Since naked mole rats live underground, where there is low gas exchange, how are they able to compensate for this and survive? 2. Explain the eusociality of naked mole rats. What advantages and disadvantages does it provide? 3. How do naked mole rats retain their heat especially if unlike other rats, they do not have fur? 4. How can non-breeding naked mole rats affect a population both negatively and positively? 5. Why do naked mole rats play a major role in the study of diseases?
Naked Mole Rat
by Angela ZhangClassification/Diagnostic characteristics
The naked mole rat, known more formally as heterocephalus glaber, is a member of the chordata animal phylum. Getting more specific, within the chordata are chordates. Looking at all chordates in general first, they, at some point in their lives, possess pharyngeal slits, a hollow dorsal nerve cord, a postanal tail, and notochord. It is important to understand what all of these characteristics are and what function they hold in order to determine what animals are chordates. The pharyngeal slits are a number of slits that the pharynx develops around and these slits open up to allow water to exit and enter. These slits are often present at some developmental stage of chordates but are often lost in adults. The hollow dorsal nerve chord is a hollow dorsal chord that connects to the notochord. The postanal tail is a tail that extends beyond the anus and finally, the notochord is the most distinctive derived chordate trait. It is composed of a core of large cells with congested fluid-filled vacuoles which make it rigid but flexible. Now within the chordates there are three clades: the cephalochordates, urochordates, and the vertebrates. The naked mole rat is classified as a vertebrate and therefore it does not have pharyngeal slits. What also makes vertebrates unique from cephalochordates and urochordates is that a dorsal supporting structure replaces the notochord in vertebrates. This dorsal supporting structure is better known as the dorsal vertebral column, where the vertebrates take their name. Vertebrates all contain a body plan that can support large, active animals. There are four key features that characterize vertebrates. The first key feature is the anterior skull with a large brain. This anterior skull contains the brain and many sensory organs. The second key feature is the rigid internal skeleton supported by the vertebral column. The third key feature is the internal organs suspended in a coelom. The coelom is a body cavity that develops within the mesoderm. The fourth feature is a well-developed circulatory system, driven by contractions of a ventral heart. (1, pg 485-488)
Domain: Eukarya
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Bathyergidae
Genus: Heterocephalus
Species: Heterocephalus Glaber
(8)
Relationship to humans
Both naked mole rats and humans are chordates. While they both at some stage in their development possessed a dorsal hollow nerve chord, a postanal tail, and notochord, a major, common characteristic that humans and naked mole rats share is that both of these species lose their notochords and later develop dorsal vertebral columns. Both the naked mole rat and humans are in fact vertebrates. They both possess the four key features all vertebrates share (mentioned in the section above) and allow for both the mammals to have similar body functions. (1, pg 487) Naked mole rats have the longest lifespan of all rodents, typically living up to 30 years; this means they are very resistant to most diseases, including cancer. Scientists are sequencing the naked mole rat genome in order to search for both disease resistant genes and genes that enable them to live for so long relative to other rodents. This studies done on naked mole rats can go to help provide new information for potential ways to further the lifespan of other species such as humans. (3)
Habitat and niche
Naked mole rats are the most eusocial mammals, mammals that show an advanced level of social organization. Naked mole rat colonies live underground in elaborate tunnel systems which are dug up by the colony members. These tunnels can extend as far as five kilometers and include up to seventy to eighty individuals with only one reproductive female and a few reproductive males. The other colony members are the offspring of the single reproductive female and are sterile workers that dig and maintain the tunnels, guard against intruders, harvest food, and use their feces to feed the queen and her offspring. (1, pg 817) A large population of the naked mole rats species live underground in arid savannah grasslands near the equator. (8)
Predator avoidance
One way naked mole rats are able to defend themselves from predators is use of their niches. Being able to build their colonies underground gives them an extreme advantage against predators as they are harder to find and harder to get to. Also, because naked mole rats live in groups, they are able to protect each other in a highly efficient way. In the large groups, the younger offspring tend to settle near their mothers as they protect the young. Another key defense technique is alarm calling. Naked mole rats are able to make a distinct noise that warns the other members of the group of a predator. However, this technique does have a high risk cost as the noise attracts attention to individuals and makes them very prominent. (1, pg 817-818)
Nutrient acquisition
Because naked mole rats developed to have jaws, they gained an advantage in improving their feeding efficiency. Animals with jaws can grasp, subdue, and swallow large prey. Another benefit naked mole rats possess is their ability to chew their food. Being able to chew food aids in chemical digestion and improves naked mole rats’ abilities to extract nutrients from their food. Also, because naked mole rats are mammals, the females have mammary glands that are able to secrete a nutritive fluid that can feed newborns. (1, pg 488, 497)
Reproduction and life cycle
The reproduction process for naked mole rats is very similar to that of humans. It requires a male and a female to mate and if successful, an offspring will result. In a large colony of naked mole rats though, only one female is reproductive and a few males are reproductive. This limits the variety of genetics within the colony as all of the offspring is from a single female. However, there are some benefits that come to individuals that cannot reproduce in the colony. When chances of individual reproduction is close to zero, an individual can best maximize its inclusive fitness by staying with and helping maintain the colony. (1, pg 817) Differences between breeding and non-breeding naked mole rats, including more exaggerated reproductive organs, are caused by hormone levels; higher lutenizing hormone levels allow the breeding individuals in the colony to become fertile. However, if a non-breeding individual were to leave the colony, their hormone levels will regulate and they could become fertile in as few as eight days. (5)
Growth and development
Naked mole rats possess key features that are most apparent in the early developmental stages, especially in the larvae. The first key feature is dorsal hollow nerve, postanal tail, and notochord, mentioned in section one. Both the dorsal hollow nerve and post anal tail are retained in the adult naked mole rate but the notochord turns into the vertebral column which becomes the new primary supporting structure. (1, pg 485-487) A unique way to identify non-breeding naked mole rats from breeding naked mole rats, is that non-breeding naked mole rats are smaller than breeding naked mole rats for both sexes (6).
Integument
Naked mole rats are often clearly recognized by the wrinkly, thin skin they have; they do not possess a hard exterior. (1, pg 817) Naked mole rats also do not possess fur, which leaves them with a lack of insulation. Naked mole rats make up for this though by having a thicker epidermal layer than regular mole rats. (6)
Movement
Naked mole rats often remain within the colony they help to build. These colonies consist of up to seventy to eighty naked mole rats that are constantly moving about in order to complete various tasks. Naked mole rats have two arms and two legs and travel on all fours throughout the tunnels in their colonies. (1, pg 817)
Sensing the environment
Naked mole rats have pain sensors in the form of the NaV1.7 ion channel similar to other mammals such as mice and humans. However, they differs slightly in that they possess three different amino acids than all other mammals, and their ion channels are made up of proteins from these amino acids. Naked mole rats are just as sensitive to sunlight and heat as mice are, but this slight variation in the NaV1.7 ion channel makes naked mole rats less sensitive to other stimulus, for example acid concentration. (4)
Gas exchange
Naked mole rats tend to burrow which causes gas exchange with the surface to be poor. The animals have to deal with low amounts of oxygen and high amounts of carbon dioxide and exhaled water vapor. This is not catastrophic through, as naked mole rat brains can tolerate low or even no oxygen for 15-30 minutes. (2)
Waste removal
Since naked mole rats have anuses, they are able to digest their food and then empty their waste through their exterior end. (1, pg 486) Nitrogen buildup in the blood is filtered by the liver, and urea is excreted out of the body. (11)
Environmental physiology (temperature, water and salt regulation)
Since naked mole rats are mammals, they have sweat glands which are able to secrete sweat that evaporates and thereby cools an animal. This allows the naked mole rats to be able to regulate their internal temperatures given the temperature of their environment. (1, pg 497)
Though naked mole rats do have sweat glands, they have less sweat glands and a thicker epidermal layer than regular mole rats in order to retain more heat since they lack fur as mentioned in the integument section. (6) Unlike most mammals, naked mole rats are essentially cold-blooded, meaning that their body temperature conforms to that of their environment. (9)
Internal circulation
Because the naked mole rat is a vertebrate, it has a closed circulatory system. In closed circulatory systems, the blood vessels keep the circulating fluid (blood) separate from the fluid around the cells (interstitial fluid). The blood cells and large molecules are able to stay within the circulatory system, but water and some other solutes leak out of the capillaries, the smallest vessel. In all closed circulatory systems, the blood is confined within a continuous system of vessels, and the blood is pumped through those vessels by one or more muscular hearts. (1, pg 747)
Chemical control (i.e. endocrine system)
The naked mole rat's endocrine system functions similarly to that of most animals. The endocrine system functions to send chemical signals throughout the body and to other organisms. Endocrine cells secrete chemical signals to induce responses in other cells. Groups of endocrine cells form an endocrine gland. The chemical signals secreted by these cells are called hormones. Hormones secreted from a cell travel through the bloodstream and bind to the target cells. The reactions induced by specific hormones may induce short term responses, such as the fight-or-flight response, or permanent changes, such as the production of reproductive organs. (10)
Review Questions
1. Since naked mole rats live underground, where there is low gas exchange, how are they able to compensate for this and survive?
2. Explain the eusociality of naked mole rats. What advantages and disadvantages does it provide?
3. How do naked mole rats retain their heat especially if unlike other rats, they do not have fur?
4. How can non-breeding naked mole rats affect a population both negatively and positively?
5. Why do naked mole rats play a major role in the study of diseases?
References
1. Principles of Life by Hillis, Sadava, Heller, Price
2. http://www.livescience.com/37658-love-mole-rats.html
3. http://naked-mole-rat.org/about.php
4. https://www.mdc-berlin.de/36925997/en/news/archive/2011/20111220-mdc_researchers__ion_channel_makes_african
5. http://academic.reed.edu/biology/professors/srenn/pages/teaching/web_2006/mole_rat_cd_dtb/reproductive.html
6. http://www.ncbi.nlm.nih.gov/pubmed/10029182
7. http://animaldiversity.ummz.umich.edu/site/resources/Grzimek_mammals/Bathyergidae/v12_id161_con_digging.jpg/medium.jpg
8. http://www-personal.umich.edu/~cberger/syllabusfolder/animaldiversity/Heterocephalus_glaber.html
9. http://www.bbc.co.uk/nature/life/Naked_mole_rat
10. Hillis, David M., David Sadava, H. Craig Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer
Associates, 2012. Print.
11. http://ex-anatomy.org/nitro.html
13. http://stevenfweinermd.files.wordpress.com/2013/06/naked-mole-rat.jpg?w=275&h=183
14. http://ferrebeekeeper.files.wordpress.com/2013/07/06_12_digging_side.jpg